There is a very easy method to get this achievement.
During Sequence 2 Memory 6
you will be given the first Den Defence attempt and as its your first time doing it the game gives you a tutorial of sorts. You can replay from the DNA menu if you have done the memory already
When the memory begins you will be asked to place an assassin leader on the roof opposite you and in front of you, you will then be asked to place an archer unit on the roofs, do this until you are asked to place a barricade. AT THIS POINT DO NOTHING
just leave the game running and you will see your morale will continue to build up BUT no enemies will come until the barricade is placed.This will be a good time to plug your controller into your console using a micro USB cable (batteries out first) to keep your controller on as you will need to leave it idling for roughly 20-30minutes
Now leave the game here until you have built up around 500-600 morale (just so you have too much) you are now free to build the barricade which will resume the waves of enemies.
Upgrade the barricade after you have built it and then build more along the path, do so at every point you can to maximise your use which is 4 self built barricades and the last one which is pre built BUT can be upgraded like the rest. Now look at the roofs at the back and place leaders on those too, after wave 4 you will unlock riflemen.
Riflemen will be your main unit to place from now on so begin filling every space on the roofs you can with riflemen, and pressing
over archers to dismiss them so you can replace them with riflemen. You can also place riflemen behind the barricades.
The battering ram will show up at wave 19, remember you can also shoot the enemies yourself by holding
whilst the rectile is over them to help speed things up.
The battering ram may break through a few barricades but it wont make it to the end.DO NOT PRESS AT ANY TIME AS THIS FIRES THE CANNON
Here's a video